1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for forming an image representing ultrasonic characteristics of a material by scanning the material with an ultrasonic wave.
2. Description of the Related Art
As conventional examples of such an apparatus, there are ultrasonic microscopes and ultrasonic probes, which are arranged to detect a reflected ultrasonic wave from or a transmitted ultrasonic wave through a material scanned with an ultrasonic wave and to form an ultrasonic image on the basis of the detected signal. In general, the apparatuses are exclusively used to know the internal acoustic (ultrasonic) characteristics of an opaque body, and the detected signals are displayed as a two-dimensional image representing the internal acoustic characteristics of the body.
In such imaging apparatuses, however, whether the reflected wave or the transmitted wave is utilized, the reflected wave from the interior of a material or the transmitted wave through the interior of the material is subject to phase distortion due to acoustic ununiformity of regions through which the ultrasonic wave travel. Therefore, simply detecting the reflected wave or transmitted wave by means of an ultrasound receiver in an attempt to form an ultrasonic image of the material may cause incident positions in the receiver on which ultrasonic waves fall to be varied due to a phase distortion. Therefore, the wavefront shape of the radiating wave and that of the incident wave become different at the transducer plane. This will adversely affect the obtained images. The effect of such phase distortion on the image is such that the contrast resulting from variations of direction of refraction of ultrasonic waves at a surface of the material, particularly at an uneven surface thereof, and the contrast produced by signals from the interior of the material appear simultaneously on an ultrasonic image in a mixed manner. It thus becomes difficult to understand the internal state of the material accurately. Moreover, where the surface state and the internal state of the material are compared, the accurate comparison becomes difficult.